From 3203ed5f0182e7dbf1c74e6598e718641a3d4013 Mon Sep 17 00:00:00 2001
From: Tim Shen <timshen@google.com>
Date: Sun, 27 Nov 2016 00:32:04 +0000
Subject: [PATCH] re PR libstdc++/78441 ([variant] variant_alternative doesn't
 allow cv qualifiers)

	PR libstdc++/78441
	* include/std/variant: Propagate cv qualifications to types returned
	by variant_alternative.
	* testsuite/20_util/variant/compile.cc: Tests.

From-SVN: r242892
---
 libstdc++-v3/ChangeLog                            |  7 +++++++
 libstdc++-v3/include/std/variant                  | 12 ++++++++++++
 libstdc++-v3/testsuite/20_util/variant/compile.cc |  9 +++++++++
 3 files changed, 28 insertions(+)

diff --git a/libstdc++-v3/ChangeLog b/libstdc++-v3/ChangeLog
index d663f70962f..4a26191fb8f 100644
--- a/libstdc++-v3/ChangeLog
+++ b/libstdc++-v3/ChangeLog
@@ -1,3 +1,10 @@
+2016-11-26  Tim Shen  <timshen@google.com>
+
+	PR libstdc++/78441
+	* include/std/variant: Propagate cv qualifications to types returned
+	by variant_alternative.
+	* testsuite/20_util/variant/compile.cc: Tests.
+
 2016-11-23  Felix Morgner  <felix.morgner@gmail.com>
 	    Jonathan Wakely  <jwakely@redhat.com>
 
diff --git a/libstdc++-v3/include/std/variant b/libstdc++-v3/include/std/variant
index 7d935753728..34ad3fdefb0 100644
--- a/libstdc++-v3/include/std/variant
+++ b/libstdc++-v3/include/std/variant
@@ -85,6 +85,18 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     using variant_alternative_t =
       typename variant_alternative<_Np, _Variant>::type;
 
+  template<size_t _Np, typename _Variant>
+    struct variant_alternative<_Np, const _Variant>
+    { using type = add_const_t<variant_alternative_t<_Np, _Variant>>; };
+
+  template<size_t _Np, typename _Variant>
+    struct variant_alternative<_Np, volatile _Variant>
+    { using type = add_volatile_t<variant_alternative_t<_Np, _Variant>>; };
+
+  template<size_t _Np, typename _Variant>
+    struct variant_alternative<_Np, const volatile _Variant>
+    { using type = add_cv_t<variant_alternative_t<_Np, _Variant>>; };
+
   constexpr size_t variant_npos = -1;
 
 _GLIBCXX_END_NAMESPACE_VERSION
diff --git a/libstdc++-v3/testsuite/20_util/variant/compile.cc b/libstdc++-v3/testsuite/20_util/variant/compile.cc
index 2470bcc7f9e..e3330bef928 100644
--- a/libstdc++-v3/testsuite/20_util/variant/compile.cc
+++ b/libstdc++-v3/testsuite/20_util/variant/compile.cc
@@ -330,3 +330,12 @@ void test_adl()
    variant<X> v8{allocator_arg, a, in_place_type<X>, il, x};
    variant<X> v9{allocator_arg, a, in_place_type<X>, 1};
 }
+
+void test_variant_alternative() {
+  static_assert(is_same_v<variant_alternative_t<0, variant<int, string>>, int>, "");
+  static_assert(is_same_v<variant_alternative_t<1, variant<int, string>>, string>, "");
+
+  static_assert(is_same_v<variant_alternative_t<0, const variant<int>>, const int>, "");
+  static_assert(is_same_v<variant_alternative_t<0, volatile variant<int>>, volatile int>, "");
+  static_assert(is_same_v<variant_alternative_t<0, const volatile variant<int>>, const volatile int>, "");
+}